UV project from view: Can foreground items block projected image from proceeding?
If I understand correctly, what you are looking for is to use an object as a mask. There are many ways to do it. Here are some:

In cycles, you can use holdout as a material on the monkey to get you what you want.



Rendered image





Also if you use a material index on the monkey's material you can substract the object from the background using the compositor.

The plane and the monkey have different materials.



On the monkey's material change the pass index to 1.



Enable material index on your render layers.



In the compositor use the IndexMA as alpha for your image.



To get this:



In Blender internal you can set the material of the monkey to be used as mask.